This invention relates to systems for transmission and reception of digital video over packet switched networks, and more particularly to video signal encoding responsive to network feedback.
In earlier work it has been shown that a promising approach for transporting real-time video traffic in packet switching networks is to use feedback from the network to modulate the source rate of a video encoder. However, a major problem exhibited by these earlier schemes is unfairness in the service provided to usersxe2x80x94where the grade of service is defined in terms of the quality of the video signal delivered to the user.
Two distinct approaches are currently utilized for supporting real-time video over packet-switched networks. One approach is for the video encoder to produce a constant bit rate data stream with the network offering guarantees of constant bandwidth to each video connection. An alternative approach is to allow the encoder to generate a variable bit rate data stream, thereby allowing for more efficient use of network resources due to statistical multiplexing gains. Simple variable bit rate encoders operate in an open loop mode in which the value of the quantizer step size is kept constant, thereby assuring a constant quality video signal to end users, as long as there is no data loss. However, network overloads can cause data loss due to buffer overflows or excessive delays with a consequent degradation in image quality.
The degradation in image quality during overloads can be controlled by modulating the source bit rate of a video encoder based on the state of the network. This results in more efficient use of the network bandwidth and a more graceful degradation in image quality during periods of congestion. However, a drawback is that during periods of overload, sources with more complex image sequences see a greater reduction in image quality. A divergence in image quality of more than 5 dB between sources has been observed, resulting in clearly visible differences in the picture quality. This unfairness occurs because while more complex image sequences require a greater amount of bandwidth to obtain the same level of quality, the feedback control schemes attempt to achieve parity in the bandwidth used by each source.
There is a need for a new feedback control scheme that provides significantly greater fairness while still allowing for efficient usage of network bandwidth and graceful degradation in image quality under overloads.
In accordance with the present invention, an apparatus and a method provides fair bandwidth sharing by adjusting video image quality in a data packet network. The present invention comprises a network load detection means for detecting a network load and a video encoding control circuit. The network load has an uncongested state, a loaded state, and a congested state. The video encoding control circuit adjusts a video quality to a target video quality, by increasing the video quality when the network load is in the uncongested state and decreasing the video quality when the network load is in the congested state.
In an enhancement of the present invention, video quality is determined as a peak mean squared error between an uncompressed image and a corresponding decoded image.
In a further enhancement of the present invention, the network load is detected by using a forward explicit congestion notification bit.
A method in accordance with the present invention for allocating a data channel in a cable network is also described.